In a radio communication system including multiple base stations, which manage multiple cells, when a mobile terminal moves from a cell to which it is connecting to another cell, processing for switching cells to connect to is performed, which is known as handover (hereinafter, abbreviated to HO as appropriate), whereby the mobile terminal can continue communication between cells. To implement handover of a mobile terminal, first, a base station that manages a cell being connected to (hereinafter, referred to as a source cell) instructs the mobile terminal to transmit a measurement report when a predetermined event occurs. Predetermined events include, for example, radio quality degradation in the source cell and the like. The measurement report by the mobile terminal includes a result of measurement of radio quality in the source cell and a group of its neighboring cells. The base station, when receiving the measurement report from the mobile terminal, determines a cell to be switched to (hereinafter, referred to as a target cell) based on the measurement report and starts a handover procedure including signaling with the mobile terminal and the target cell.
Examples of an event that may trigger the transmission of a measurement report are provided in NPL 1 by LTE (Long Term Evolution)/E-UTRAN (Evolved UTRAN). The substantial part of a reporting event, which is provided as Event A3 (Neighbor becomes offset better than serving) in NPL 1, is represented by the following expression (1):Ps+Os<Pt+Ot  (1)where Ps is a result of measurement of radio quality in the source cell, Pt is a result of measurement of radio quality in a neighboring cell, Os is an offset value to the radio quality in the source cell, and Ot is an offset value to the radio quality in the neighboring cell.
In LTE, Ps and Pt are the received power of a downlink reference signal, RSRP (Reference Signal Received Power), or the received quality of a reference signal, RSRQ (Reference Signal Received Quality). RSRQ is the ratio of RSRP to gross received power, RSSI (Received Signal Strength Indicator). Moreover, Os in the expression (1) acts on the radio quality of a downlink reference signal in the source cell, while Ot acts on the radio quality of a downlink reference signal in the neighboring cell.
Ot is a handover parameter generally referred to as a cell individual offset CIO, and a different offset value can be set for each neighboring cell registered in a neighbor list. CIOs are included in the neighbor list, which is notified by a base station to a mobile terminal that is connecting to a cell managed by this base station.
When an operation condition of the expression (1) is set on a base station, the operation condition of the expression (1) is notified to a mobile terminal that is connecting to a cell managed by the base station. If Ps and Pt, which are results of measurement of the radio quality in the source cell and a neighbor cell, respectively, satisfy the condition of the expression (1), the mobile terminal, triggered by this fact, transmits a measurement report to the base station managing the source cell.
The base station, when receiving the measurement report from the mobile terminal, determines a target cell based on the measurement report and starts processing for handover to the target cell. In this event, if the start of handover is too late, the radio quality in the source cell falls below required quality before the handover to the target cell is completed, resulting in an abnormal disconnection of communication. On the other hand, if the start of handover is too early, the radio quality in the target cell falls below required quality immediately after the handover to the target cell is completed, resulting in an abnormal disconnection of communication.
In the case of a failure caused by the too late timing of handover to a target cell (hereinafter, referred to as “Too Late HO”), handover timing is advanced by increasing the CIO (i.e., the offset value Ot in the expression (1)), whereby the handover success rate can be improved. On the other hand, in the case of a failure caused by the too early timing of handover to a target cell (hereinafter, referred to as “Too Early HO”), handover timing is delayed by decreasing the CIO, whereby the handover success rate can be improved. Accordingly, it is possible to reduce handover failures by dynamically adjusting a handover parameter such as the above-described offset value Ot (CIO).
For example, PTL 1 discloses a handover optimization method in which HO failures (Too Late HOs, Too Early HOs) are reduced by dynamically adjusting HO parameters such as TTT (Time-To-Trigger) and CIO in units of cells, based on measurement reports from terminals.
According to NPL 2, three types of handover failure resulting in an abnormal disconnection of a radio link (RLF: Radio Link Failure), namely, “Too Late Handover,” “Too Early Handover” and “Handover to Wrong Cell,” are defined as follows (22.4.2). In the present description, these three types of handover failure will be treated as handover failures (HO failures).
<Too Late HO>
Too Late Handover is handover from a source cell to a target cell when a mobile terminal (UE) having experienced a RLF in the source cell during handover processing tries to reestablish a connection to the target cell (including reestablish of a radio link). Alternatively, it is a RLF occurring in the source cell when a mobile terminal having experienced a RLF in the source cell before handover is started tries to reestablish a connection to a cell different from the source cell.
<Too Early HO>
Too Early Handover is handover from a source cell to a target cell when a mobile terminal having experienced a RLF in the target cell during handover processing or immediately after handover is completed tries to reestablish a connection to the source cell.
<HO to Wrong Cell>
Handover to Wrong Cell is handover from a source cell to a target cell when a mobile terminal having experienced a RLF in the source or target cell during handover processing or immediately after handover is completed tries to reestablish a connection to a cell (neighbor) different from any of the source and target cells.